1. Field of the Invention
This invention relates generally to semiconductor manufacturing equipment, and more specifically to spindle downforce calibration using an internal load cell inside a carrier.
2. Description of the Related Art
Often, in the fabrication of semiconductor devices, there is a need to perform chemical mechanical planarization (CMP) operations. By way of background, integrated circuit devices are in the form of multi-level structures, wherein transistor devices having diffusion regions are formed at the substrate level. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define desired functional devices. In addition, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide.
As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material grows. Without planarization, fabrication of further metallization layers becomes substantially more difficult due to the variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material and metal CMP operations are performed to remove excess material.
Typically, a chemical mechanical planarization (CMP) system is utilized to polish a wafer as described above. A CMP system generally includes system components for handling and polishing the surface of a wafer, such as an orbital polishing pad, or a linear belt polishing pad. The pad itself often is made of a polyurethane material or polyurethane in conjunction with other materials such as, for example a stainless steel belt. In operation, the belt pad is put in motion and a slurry material is applied and spread over the surface of the belt pad. Once the belt pad having slurry on it is moving at a desired rate, the wafer is lowered onto the surface of the belt pad. In this manner, wafer surface that is desired to be planarized is substantially smoothed, much like sandpaper may be used to sand wood. The wafer may then be cleaned in a wafer cleaning system.
FIG. 1A shows a linear polishing apparatus 10, which is typically utilized in a CMP system to polish away materials on a surface of a semiconductor wafer 16. The material being removed may be a substrate material of the wafer 16 or one or more layers formed on the wafer 16. Such a layer generally includes one or more of any type of material formed or present during a CMP process such as, for example, dielectric materials, silicon nitride, metals (e.g., aluminum and copper), metal alloys, and semiconductor materials.
In operation, the linear polishing apparatus 10 utilizes a polishing belt 12, which moves linearly in respect to the surface of the wafer 16. The belt 12 is a continuous belt rotating about rollers 20, which are typically driven by a motor so that the rotational motion of the rollers 20 causes the polishing belt 12 to be driven in a linear motion 22 with respect to the wafer 16.
The wafer 16 is held by a wafer carrier 18, generally using a mechanical retaining ring and/or by vacuum. The wafer carrier 18 positions the wafer atop the polishing belt 12 and moves the wafer 16 down to the polishing belt 12, applying the wafer 16 to the polishing belt 12 with pressure such that the surface of the wafer 16 is polished by a surface of the polishing belt 12. Typically, the wafer carrier 18 is part of a spindle drive assembly 30 (shown in FIG. 1B) that enables application of polishing pressure to the wafer 16.
FIG. 1B shows a conventional spindle drive assembly 30 that may be utilized to apply the wafer 16 to the polishing belt in the CMP apparatus 10 (as shown above in FIG. 1A). The spindle drive assembly 30 includes the wafer carrier 18 connected to a spindle 42. The spindle 42 is attached to a force magnifier 34, which is connected to a hinge 40 and an air cylinder 32. Typically, the force magnifier 34 is a machined aluminum arm that acts in a manner similar to a lever such that force applied by the air cylinder 32 is magnified onto the spindle 42. The spindle 42 then pushes down the wafer carrier 18, known as applying downforce, which in turn applies pressure to the wafer 16 for polishing action (as shown in FIG. 1A).
During a planarization process, the wafer removal rate profile is highly dependent on the wafer carrier 18 downforce pressure exerted by the spindle 42. Thus, if the wafer carrier 18 downforce pressure is too high for a particular process, the overall effect of the polishing is diminished. Hence, the downforce pressure exerted by the spindle 42 is generally calibrated prior to use of the CMP system.
FIG. 1C is diagram showing a prior art spindle force downforce calibration system 50. As shown in FIG. 1C, the spindle force downforce calibration system 50 attaches to the wafer carrier 18, and includes an alignment plate 54 positioned below a polishing plate 52, and a load cell holder 56 that holds a button load cell 58 above the polishing belt 12. In normal operation, the polishing plate 52 is used to hold a wafer during the polishing process. However, during calibration, the alignment plate 54 is positioned below the polishing plate 52 to align placement of the load cell holder 56. In particular, the alignment plate 54 includes a beveled center 60, in which the top portion load cell holder 56 is inserted. In this manner, the load cell holder 56 can be positioned in the center of the carrier 18 to equalize the downforce measured.
The load cell holder 56 holds the button load cell 58, which is used to measure downforce for calibration. As mentioned above, the load cell holder 56 is located above the polishing belt 12 and below the alignment plate 54. Thus, in operation, the spindle drive assembly 42 applies downward force to the wafer carrier 18, which in turn transfers the downforce to the button load cell 58 via the alignment plate 54 and the load cell holder 56. In this manner, the button load cell 58 measures the downforce at the center of the polishing plate 52. Typically, the button load cell 58 is attached to a hand-held meter, which can be read to determine the measured downward force.
Unfortunately, the prior art spindle force downforce calibration system 50 requires the wafer carrier 18 to be positioned approximately three inches above the polishing belt 12. Since the wafer carrier 18 is not located at the position of the polishing belt during calibration, the measured downforce will be different than what is actually applied during normal CMP operation. More specifically, linkage is used to apply downforce to the spindle drive assembly 42. However, the force applied by the linkage, or force amplifier, varies depending on the position of the linkage when the downforce is applied. The linkage has a different position when the wafer carrier 18 is located three inches above the polishing belt 12 than when the wafer carrier 18 is positioned approximately at the same level as the polishing belt 12. As a result, 20-30 psi calibration errors can occur during calibration using the prior art spindle force downforce calibration system 50.
In view of the foregoing, there is a need for a CMP calibration system capable of providing accurate calibration results. Hence, the CMP calibration system should measure downforce during calibration at approximately the wafer carrier height used during normal CMP operation.
Broadly speaking, the present invention fills these needs by providing a spindle drive downforce calibration apparatus that includes a load cell located within the calibration wafer carrier. In one embodiment, a method is disclosed for spindle downforce calibration. A calibration wafer carrier is provided having a calibration load cell recess. A calibration load cell is positioned within the calibration load cell recess. Downward force is applied to the calibration wafer carrier, while the calibration wafer carrier is positioned substantially at a polishing height. In this case, the polishing height is substantially equivalent to a height of a normal wafer carrier during a chemical mechanical polishing (CMP) operation. The downward force is then measured using the calibration load cell.
In an additional embodiment, a system for spindle downforce calibration is disclosed. The system includes a calibration wafer carrier having a calibration load cell recess, and a calibration load cell disposed within the calibration load cell recess. The system further includes a meter in communication with the calibration load cell. In operation, the calibration load cell measures a downward force applied to the to the calibration wafer carrier. The measurement can be taken when the calibration wafer carrier is positioned substantially at a polishing height, which is substantially equivalent to a height of a normal wafer carrier during a chemical mechanical polishing (CMP) operation. In one aspect, the measured downward force is compared to a tool downward force measurement that is measured using a tool load cell, which is generally utilized to measure tool downward force during a normal CMP process.
A calibration wafer carrier for spindle downforce calibration is disclosed in a further embodiment of the present invention. The calibration wafer carrier includes a calibration load cell recess, and a calibration load cell disposed within the calibration load cell recess. The calibration load cell is capable of comparing resistances from stretched metal layers to measure downward force. In addition, the calibration wafer carrier includes a downforce meter connector that is coupled the calibration load cell and to a meter. En operation, the calibration load cell measures a downward force applied to the to the calibration wafer carrier when the calibration wafer carrier is positioned substantially at a polishing height, which is substantially equivalent to a height of a normal wafer carrier during a chemical mechanical polishing (CMP) operation. In one aspect, the calibration load cell can provide the calibration data to the meter, which in one embodiment can be a hand-held meter. Also, the calibration wafer carrier generally replaces a normal wafer carrier during a calibration operation.
By locating the calibration load cell within the calibration wafer carrier, the spindle drive downforce can be measured at the polishing height, which is the normal wafer carrier height during a CMP process. As a result, higher measurement accuracy is achieved. In addition, calibration time is greatly reduced using the embodiments of the present invention because the spindle drive downforce calibration system is self-contained and does not require the placement of various loose calibration components beneath the wafer carrier. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.